An electric power steering apparatus (EPS) which provides a steering mechanism of a vehicle with a steering assist torque (an assist torque) by means of a rotational torque of a motor, applies a driving force of the motor as the steering assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate the steering assist torque, such a conventional electric power steering apparatus performs a feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a steering assist command value (a current command value) and a detected motor current value becomes small, and the adjustment of the voltage applied to the motor is generally performed by an adjustment of duty command values of a pulse width modulation (PWM) control.
A general configuration of the conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft) 2 connected to a steering wheel (handle) 1, is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack and pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. Further, the column shaft 2 is provided with a torque sensor 10 for detecting a steering torque Th of the steering wheel 1, and a motor 20 for assisting the steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. Electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of an assist command with an assist map based on a steering torque Th detected by the torque sensor 10 and a vehicle speed Vel detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 based on a voltage control value Vref obtained by performing compensation and so on with respect to the current command value. A steering angle sensor 14 is not indispensable and may not be provided. It is possible to obtain the steering angle θfrom a rotational sensor such as a revolver connected to the motor 20.
A controller area network (CAN) 40 to send/receive various information and signals on the vehicle is connected to the control unit 30, and it is also possible to receive the vehicle speed Vel from the CAN 40. Further, a Non-CAN 41 is also possible to connect to the control unit 30, and the Non-CAN 41 sends and receives a communication, analogue/digital signals, electric wave or the like except for the CAN 40.
In such the electric power steering apparatus, the control unit 30 mainly comprises a central processing unit CPU (or a micro-processing unit MPU or a micro-controller unit MCU), and general functions performed by programs within the CPU are shown in FIG.2. As shown in FIG.2, the steering torque Th detected by the torque sensor 10 and the vehicle speed Vel detected by the vehicle speed sensor 12 are inputted into a current command value.
A deviation I (=Irefm-Im) being a subtraction result at the subtracting section 32B is inputted into a PI-control section 35, the PI-controlled voltage command value Vref is inputted into a PWM-control section 36 and is calculated duty ratios, and the motor 20 is PWM-driven through an inverter 37 with PWM-signals. The motor current value Im of the motor 20 is detected by a motor current detector 38 and is fed back to the subtracting section 32B. A rotational sensor 21 such as a revolver is connected to the motor 20 and the steering angle θis outputted.
The compensating section 34 adds a self-aligning torque (SAT) 343 detected or estimated with an inertia 342 at an adding section 344, further adds the result of addition performed at the adding section 344 with a convergence 341 at an adding section 345, and then outputs the result of addition performed at the adding section 345 as the compensation signal CM thereby to improve the characteristic of the current command value.
As mentioned above, the conventional electric power steering apparatus generally generates a control signal for controlling the motor with the PI-control in a current control section, and a PI-control gain (a proportional gain and an integral gain) is suitably adjusted corresponding to a kind of the vehicle.
When the gain of the PI-control becomes great, a noisy sound and a vibration due to a noise or the like occur. For example, in a case that the motor is a brush motor and a steering holding operation to hold a constant steering angle is performed by applying the steering torque to the handle, a motor current becomes discontinuous due to a variation of impedance between a commutator and a brush. This action becomes a trigger, the current variation occurs, and then the vibration to the handle and the noisy sound are generated. In this connection, it is necessary to restrict the PI-control gain so as not to occur the vibration and the noisy sound. However, if the PI-control gain is restricted as stated above, a frequency characteristic of the current control is abated and there is a problem that it is difficult to improve a responsibility of the assist control.
Further, even if the PI-control gain is efficiently abated, it is perfectly impossible to prevent the vibration in the vicinity of a resonant frequency of the steering system and a comfortable steering feeling is not always obtained.
The electric power steering apparatus for solving the above problem is proposed in Japanese Published Unexamined Patent Application No. 2006-188183 A (Patent Document 1). The electric power steering apparatus disclosed in Patent Document 1 comprises a vibration detecting section to detect the vibration of the handle, a continuing time that the vibration detecting section is detecting the vibration is counted by a continuing time counting section and the gain setting section variably sets the gain of the PI-control section. The vibration detecting section detects due to the motor angular velocity and the steering torque whether the vibration occurs at the handle or not.